The invention relates generally to electrical systems for providing electrical power to utility grids at interconnection points. More specifically, the invention relates to an electrical system which converts electrical power generated by renewable energy sources such as wind turbines and photovoltaic modules into electrical power which meets a low harmonic requirement of the interconnection point.
Recently, renewable energy sources, such as wind turbines, have received increased attention as environmentally safe and relatively inexpensive alternative energy sources. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient. To maximize the efficacy of power generation and to simplify connection to the utility grid, wind turbines are often located in proximity to one another, which are generally referred to in the pertinent arts as a “wind farm.” Wind turbines in the wind farm generate electrical power and feed electrical current into the utility, such as a power grid, at a point of common connection (PCC).
A type of wind turbine that keeps the rotational generator speed proportional to the wind speed is a variable speed wind turbine. Examples of the variable speed wind turbine are described in, for example, Richardson et al., U.S. Pat. No. 5,083,039. The variable speed wind turbine includes a generator, a converter on the generator side, a DC link Bus, and a converter connected to the grid. The variable frequency energy of the generator is transferred to the DC link bus by the generator side converter and later converted to a fixed frequency by the grid side converter.
The grid side active converter typically utilizes semiconductor switches, such as insulated gate bipolar transistors (IGBTs), to convert DC voltage to AC voltage at the grid frequency by pulse width modulated switching actions of those semiconductor switches. The switching actions tend to create undesirable harmonics. To avoid the problems caused by these harmonics, a number of filters are installed. The filter components are typically bulky and expensive.
There is a need in the art to provide an improved electrical system for a wind farm which can meet the low harmonics requirement at an electrical connection point for a group of wind turbines which differs from conventional systems. Such systems would additionally be useful for other renewable energy sources such as photovoltaic systems, for example.